As is known, many liquid or pourable food products, such as fruit juice, UHT (ultra-high-temperature treated) milk, wine, tomato sauce, etc., are sold in packages made of sterilized packaging material.
A typical example is the parallelepiped-shaped package for liquid or pourable food products known as Tetra Brik Aseptic (registered trademark), which is made by folding and sealing laminated strip packaging material. The packaging material has a multilayer structure comprising a base layer, e.g. of paper, covered on both sides with layers of heat-seal plastic material, e.g. polyethylene. In the case of aseptic packages for long-storage products, such as UHT milk, the packaging material also comprises a layer of oxygen-barrier material, e.g. an aluminium foil, which is superimposed on a layer of heat-seal plastic material, and is in turn covered with another layer of heat-seal plastic material forming the inner face of the package eventually contacting the food product.
Packages of this sort are normally produced on fully automatic packaging units, on which a continuous tube is formed from the web-fed packaging material; the web of packaging material is sterilized on the packaging unit, e.g. by applying a chemical sterilizing agent, such as a hydrogen peroxide solution, which, once sterilization is completed, is removed from the surfaces of the packaging material, e.g. evaporated by heating; the web so sterilized is then maintained in a closed, sterile environment, and is folded and sealed longitudinally to form a tube, which is fed along a vertical advancing direction.
In order to complete the forming operations, the tube is filled with the sterilized or sterile-processed food product, and is sealed and subsequently cut along equally spaced transversal cross sections.
Pillow packs are so obtained, which have a longitudinal sealing band and a pair of top and bottom transversal sealing bands.
Packaging units are known, as described for example in European Patent EP-B-0887265, which comprise two chain conveyors defining respective endless paths.
In greater detail, the first conveyor is fitted with a plurality of first jaws, each comprising a respective sealing element and a respective extractable cutting element.
The second conveyor is fitted with a plurality of second jaws, each associated to a relative first jaw and comprising a respective counter-sealing element and a respective seat.
In particular, the sealing element is a heating element and the counter-sealing element is made of elastomeric material, which provides the necessary mechanical support to grip the tube to the required pressure.
The paths of the chain conveyors comprise:                respective operative branches substantially facing and parallel to each other, and between which the tube of packaging material is fed, so that the first jaws cooperate with the second jaws on the other conveyor to grip the tube at a number of successive cross sections and to heat-seal the tube and cut the packs; and        respective return branches, along which the first jaws and the second jaws are spaced apart from the tube.        
In greater detail, the first jaws and the corresponding second jaws at first move away from each other and then move towards each other, as they move along return branches downstream of the operative branches with respect to the advancing direction of the chain conveyors.
As each first jaw cooperates with the corresponding second jaw along the operative branches, the respective sealing element cooperates with the respective counter-sealing element to heat-seal the tube at a corresponding transversal section.
Furthermore, as each first jaw cooperates with the corresponding second jaw, the respective cutting element is extracted to cut the formed sealed packs at the transversal section so as to form corresponding sealed packages.
Each first jaw and second jaw also comprises respective forming half-shells, which are hinged with respect to the relative sealing element or the counter-sealing element, so as to control the volume of the relative packs in formation.
The half-shells of each first jaw and of the corresponding second jaw move cyclically between:                an open position, in which they are detached from the tube; and        a closed position, in which they contact the tube and fold the portion of the tube between two consecutive sealing sections to define and control the volume of the packs being formed.        
In this way, as the sealing element of each first jaw seals the pack in formation, the half-shells of the same first jaw and of the corresponding second jaw control the volume of the pack in formation.
More specifically, the half-shells may be spring-loaded by respective springs into the respective open position, and have respective rollers, which cooperate with respective cams designed to move the half-shells into the respective closed position by the time the forming assembly reaches a predetermined position as it moves down.
Each half-shell has a C-shaped cross section, and comprises, integrally: a main wall, and two parallel lateral flaps projecting towards the axis of the tube of packaging material from respective opposite end edges of the main wall.
In the closed position, the main walls are located on opposite sides of the tube axis, are parallel to each other, and cooperate with respective first portions of the tube.
In the closed position, the flaps of one half-shell cooperate with respective second portions of the tube to completely control the volume of the pack being formed, and, on the opposite side to the relative main wall, face corresponding flaps on the other half-shell.
Finally, the packaging unit comprises a pair of fixed cam assemblies arranged on respective sides of the tube.
The cam assemblies cooperate in rolling manner with respective rollers carried by the first jaws and the second jaws.
The profiles of the cam assemblies are so designed to move the first jaws and the second jaws along the respective return branches and operative branches, and to ensure the smoothest transition between the various positions assumed by the first jaws and the second jaws, so as to prevent undesired stress on the materials.
The cam assemblies are shaped so as to displace, after the completion of the formation of the tube, the half-shells from the closed position to the open position, as they travel parallel to the advancing direction of the tube.
In this way, the cut and sealed packs fall, under the gravity action, towards an outlet conveyor, which is arranged in the lower region of the packaging unit and along the advancing direction of the tube.
Though performing excellently on the whole, the packaging units of the type described still leave room for further improvement.
In particular, the Applicant has found that there is the risk that the packaging material of a formed packs can stick to the packaging material intended to form the immediately upstream pack in formation, thus creating connecting bridges between two consecutive packs.
As a result, there is the risk that the gravity action is not able to ensure a proper, repeatable and precise detachment of the packs and, therefore, transport of the packs towards the outlet conveyor.
A need is felt within the industry to improve the repeatibility and the precision of the detachment of the packs and, therefore, transport of the formed packs towards the outlet conveyor.